With remarkable developments of information processing technologies, semiconductor light emitting devices have increasingly large demands as light sources for optical communications, optical recording media, and image displays. In particular, visible light emitting diodes (hereinafter, a light emitting diode will be abbreviated to LED) of higher intensities are strongly required for development thereof as effective light sources for image displays.
AlGaInP based mixed-crystal semiconductor lattice matched with a GaAs substrate, as it is capable of forming excellent hetero-junctions and suited to obtain high luminance LEDs, is widely used in recent years as a material for forming high luminance LEDs which can display visible colors from red to green.
To obtain high luminance LEDs, it is necessary to generate light from a prescribed limited region thereof. Basically, conventional AlGaInP based LEDs, despite of their various systems for guiding the light from the prescribed limited region, were classified roughly into only two types based on the shapes of contact layers thereof.
One example is an LED having a contact layer of the shape as shown in FIG. 5. FIG. 5(a) is a schematic sectional view showing the layered structure of a conventional AlGaInP based LED, and FIG. 5(b) is a diagram showing the emission pattern of the conventional LED shown in FIG. 5(a).
The conventional LED 50 shown in FIG. 5 includes, on an n-type GaAs substrate 52, a double hetero-junction structure composed of an n-AlGaInP lower cladding layer 54, an active layer 56, and a p-AlGaInP upper cladding layer 58 which are consecutively layered. The LED 50 also has: a p-GaAs contact layer 60 formed on the double hetero-junction structure so as to have the same diameter as an upper electrode, and to be concentrically thereto; the upper electrode 64 formed on the contact layer 60 and having a circular central opening 62 as a light emitting region; and an n-side electrode 66 formed on the bottom surface of the GaAs substrate 52.
The other example is an LED 70 having a contact layer of the shape as shown in FIG. 6. FIG. 6(a) is a schematic sectional view showing the layered structure of the another conventional AlGaInP based LED, and FIG. 6(b) is a diagram showing the emission pattern of the conventional LED shown in FIG. 6(a).
The conventional LED 70 shown in FIG. 6 has a structure similar to that of the LED 50 shown in FIG. 5, except that a current diffusion layer 72 made of p-AlGaAs and a p-GaAs contact layer 74 having a shape similar to that of the upper electrode 64 are formed on the double heterojunction structure.
In the LED 50 of FIG. 5 having the circular light emitting region 62, the contact layer 60 made of GaAs is usually formed in order to provide lattice matching with the AlGaInP based double heterojunction structure being in lattice match with GaAs, followed by etching thereof to form a circular contact layer, and by providing thereon the ring-shaped upper electrode 64 having the circular opening as the light emitting region, to thereby obtain the circular emission pattern.
GaAs, however, has a high absorbance with respect to the emission wavelength; therefore, in the LED 50, the contact layer has a large absorbance for radiated light, resulting in a large factor of decreasing the radiation from the LED.
Meanwhile, the LED 70 shown in FIG. 6 and having the contact layer of a ring shape similar to that of the upper electrode is a conventional example which avoids the aforementioned decrease in brightness resulting from the absorption by the GaAs contact layer. Here, the high-absorptive GaAs contact layer 74 is formed at the limited area directly below the ring-shaped upper electrode 64; besides, the current diffusion layer 72 is arranged below the contact layer 74 for sufficient diffusion of the injected current.
This, while reducing the absorption of emitted light in the contact layer to obtain emission of higher intensities, leads to the drawbacks in that: the injected current diffuses beyond the upper electrode to result in a poor emission efficiency; and, as shown in FIG. 6(b), a doughnut-shaped emission pattern occurs having a blank at the center, which is not acceptable as a circular emission pattern.
Thus, as an LED for preventing the absorption in the GaAs contact layer and having an emission pattern of approximately circular shape, there is a proposal for an LED 80 having the configuration as shown in FIG. 7 in which the current diffusion layer outside the electrode is etched for removal. FIG. 7 is a schematic sectional view showing the layered structure of yet another conventional LED. This LED 80 has a configuration similar to that of the LED 70 shown in FIG. 6, except that a p-type current diffusion layer 82 made of AlGaAs is formed as a circular layer which has the same outer diameter as those of the upper electrode 64 and the contact layer 74 and is concentric thereto.
While the LED 80 shown in FIG. 7, due to suppression of the light emission outside the upper electrode, is improved in emission efficiency as compared with the LED shown in FIG. 6, it has a drawback in that its emission pattern tends to take a doughnut shape and is difficult to form in an acceptable circular shape. Besides, a step is needed for deeply etching the current diffusion layer, which yielded the drawbacks of increasing the steps in fabricating the elements and of the complicated processes.
As described above, the conventional AlGaInP based LEDs have various problems, and it is difficult, by relatively simple processes, to fabricate an LED for emitting light in a desired emission pattern and at higher intensities.